An important area of work for network technicians, and routing and internet experts, is the further development of packet-switched networks. An important objective of this further development is to be able to offer a comprehensive spectrum of services over packet-switched networks.
Apart from the conventional applications for data transmission, ever more services with real-time requirements, such as telephone (e.g. Voice over IP) and the transmission of image data in real time (e.g. video on demand, video conferences), are to be realized over packet-switched networks. From this arise new requirements which the packet-switched networks must meet. Adherence to quality characteristics for services with real-time requirements—frequently referred to in this context as the ‘quality of service’, abbreviated to QoS—is of central importance.
The packet-switched networks which are currently most popular are based on the IP (Internet Protocol) protocol. The success of these is to a large extent explained by their comparatively low complexity and high flexibility. Both of these arise from the way that packets are forwarded in the IP network.
Within IP networks, the packets are routed by reference to their IP addresses. In the large majority of cases, the routing is effected on a per-hop basis, i.e. by reference to the packet address the routers identify a destination, normally another router, to which the packet concerned is forwarded. At the end of the transmission via a chain of routers, the packet is delivered to the destination address, often a host or a gateway.
In general, no information relating to the transmission path is available to an individual router apart from details of the next hop or the next stage, as applicable. As a result, the maintenance and administration of the routing tables requires little effort. In addition, the method is flexible to the extent that alternative and default destinations can be provided for the next hop which, for example, ensure that the packet can be forwarded in the event of a malfunction or unknown addresses.
IP networks using conventional routing techniques are not very suitable for real-time traffic. Delays to packets, and their loss, are not subject to sufficiently stringent controls to be able to guarantee the quality characteristics necessary for real-time transmission.
Methods for better control of the transmission parameters include the reservation of transmission capacity for services with real-time requirements, and the specification of transmission paths in the network. The RSVP (Resource Reservation Protocol) signaling protocol was developed for the purpose of reserving bandwidth in IP networks. The RSVP protocol is used with, among others, the MPLS (Multi-Protocol Label Switching) protocol which permits the transmission path to be defined. The MPLS protocol provides for a packet, when it enters into a network, to be allocated a label by an LSR (Label Switching Router) edge router, which defines an LSP (Label Switched Path) path through the network. The packet is then forwarded by internal LSR (Label Switching Router) routers according to the LSP path defined by the label.
A choice of path is also made as part of the ATM (Asynchronous Transfer Mode) technique, and for IP networks can also be effected by means of the Source Route Option of the IP datagram (although in practice this is seldom supported).
The reservation of paths permits guarantee statements in respect of a QoS transmission, but is associated with high complexity and the loss of flexibility (by comparison with “best effort” packet-switched networks).